Common-rail injection systems are equipped with a rail pressure sensor for determining the injection pressure and used in control of the injection pressure. Said rail pressure sensor is, therefore an essential component of a common-rail injection system, which inter alia contributes to complying with maximum permissible values for the emissions that occur and the fuel consumption. A malfunction of said rail pressure sensor would result in a significant emission and drivability degradation and would constitute a safety risk.
In the event of a sensor malfunction with which too low a pressure is measured, an existing pressure control circuit would increase the injection pressure and thus the rail pressure, perhaps to a critical pressure level. As a result, mechanical components of the common-rail injection system would be overloaded, so that for example a fuel leak can occur. Consequently, it is necessary to detect the presence of a rail pressure sensor defect and place an associated fault signal in a fault memory to indicate the source of the fault to service staff and to enable suitable countermeasures to be taken.
It is already known to perform a pressure equalization with ambient pressure when the common-rail injection system is unpressurized for detecting a rail pressure sensor defect. But this type of detection can only be performed before the start of the respective engine and gives no indication of whether the pressure measurement value provided by the rail pressure sensor is correct or not in the entire measurement range, for example at a system pressure of 1500 bar. Said type of detection also provides no information about any drifting of the measurement values provided by the sensor and also no information about any characteristic curve gradient errors.
Furthermore, it is also known to carry out a redundant version of the rail pressure measurement by means of two rail pressure sensors and to continuously compare the measurement values obtained in the entire sensor measurement range with each other for detecting a defect of a rail pressure sensor. If the obtained measurement values deviate from each other, then the presence of a malfunction is detected and necessary countermeasures can be initiated. Such a redundant version of the rail pressure measurement is however difficult to implement for structural reasons and furthermore causes comparatively high costs.
An analysis of the rail pressure measurement signals provided by a rail pressure sensor is generally carried out in a control unit connected to the rail pressure sensor, to which the rail pressure measurement signals provided by the rail pressure sensor are delivered. One possibility is transmitting the rail pressure measurement signals that are provided by the rail pressure sensor to the control unit in digital form. However, the high data rates that are required by the injection system for pressure control cannot be achieved by means of such a digital transmission.
An alternative possibility is transmitting the rail pressure measurement signals provided by the rail pressure sensor to the control unit in the form of analog signals. Said analog transmission does not however give the possibility of a manipulation lock for the purpose of making difficult or preventing tuning of the rail pressure sensor to achieve an engine performance increase.
A method and an apparatus for plausibility checking the output signal of a rail pressure sensor are known from DE 10 2008 043 413 A1. With said known method, an analog signal of a rail pressure sensor characterizing the rail pressure is delivered to a control unit and processed there. Furthermore, an additional digital signal characterizing the rail pressure is output by the rail pressure sensor and is compared with the analog signal in the control unit for plausibility checking of the analog signal characterizing the rail pressure.